Although rotary wing aircraft can fly in any direction, such aircraft, and particularly unmanned rotary wing aircraft of the type having counterrotating blades and no tail rotor, are most stable when flying in the forward direction, due to a nose-heavy weight distribution which is designed into the aircraft. For improved stability, performance margin and safety, a rotary wing aircraft usually hovers with its heading directly into the wind; this is particularly true for unmanned rotary wing aircraft. If such an aircraft is given a new course which is significantly diverse from its current heading, it could fly directly in that direction regardless of the true heading of the aircraft. However, airspeed limiting, to assure safe operation, may reduce the commanded forward or lateral velocity so that the net course of the aircraft will be determined by vector addition of limited airspeeds, either one or both of which are simply established by the limiters, rather than by the desired course. Thus, any command to cause the rotary wing aircraft to advance from hover to some new destination is very likely to cause it to fly, at least initially, in the wrong direction, due to independent limiting of airspeed in the longitudinal channel and in the lateral channel.
Even if a rotary wing aircraft is yawed from having its heading into the wind during hover so as to cause its heading to be in the direction of a newly-commanded destination, the aircraft could be subjected to strong lateral winds, or a strong tailwind, which are unsafe conditions for rotary wing aircraft, depending on the severity thereof. And, when flying along a course, the wind direction or speed may change, or there may be gusts. The effect of strong winds on a rotary wing aircraft can result in saturated lateral cyclic pitch or aft longitudinal cyclic pitch, as a consequence of limiting to ensure that the aircraft stays within the safe envelope of commands, which assure aircraft stability. At times, if the aircraft is yawed so as to fly sufficiently into the wind to maintain course, it is not capable of staying within lateral cyclic pitch limits and maintaining a direction toward the desired destination.
To limit the complexity and weight of an unmanned rotary wing aircraft, it is convenient to use existing control channels to navigate the aircraft from one position to a new position. Specifically, the hover hold command channel may be utilized to command the aircraft to a new destination by simply introducing a range and bearing as errors into the hover hold system. If hover hold is implemented by means of longitudinal and lateral velocity hold channels, then a speed must be commanded to cause the aircraft to leave its initial hover position and advance to its new destination before resuming hover. If the speed is selected so that the aircraft can decelerate to hover at the new position without overshoot, the selected speed may cause the aircraft to proceed much too slowly from one position to another.